Purification of contaminated propylene oxide by extractive distillation

ABSTRACT

An extractive distillation agent consisting essentially of 2-methyl-2,4-pentanediol is fed to an extractive distillation column used for the distillation of propylene oxide contaminated with water, acetone and methanol to obtain an overhead distillate fraction consisting of essentially anhydrous propylene oxide contaminated with reduced quantities of acetone and methanol, and a heavier bottoms distillation fraction containing substantially all of the 2-methyl-2,4-pentanediol, water and acetone and some of the methanol introduced into the distillation column.

BACKGROUND OF THE INVENTION

1. Technical Field of the Invention

This invention relates to the purification of propylene oxide. Moreparticularly, this invention relates to a distillation process forremoving contaminating quantities of impurities includingoxygen-containing impurities such as methanol, acetone and water from animpure propylene oxide feedstock. Still more particularly, thisinvention relates to a method wherein an impure propylene oxidefeedstock contaminated with from about 50 to about 4,000 ppm ofmethanol, from about 0.01 to about 2 wt. % of water and from about 0.01to about 2 wt. % of acetone is purified in an extractive distillationcolumn using 2-methyl-2,4-pentanediol as an extractive distillationagent.

2. Prior Art

It is known to react a hydroperoxide feedstock such as tertiary butylhydroperoxide with propylene in the presence of an epoxidation catalystin order to provide a reaction product comprising propylene oxide, analcohol corresponding to the hydroperoxide feedstock, a solvent, andimpurities (see, for example, Kollar U.S. Pat. No. 3,350,422, KollarU.S. Pat. No. 3,351,635 and Sorgenti U.S. Pat. No. 3,666,777.

It is also known to separate the reaction product by distillation inorder to obtain a plurality of fractions including, for example, apropylene recycle fraction, a propylene oxide product fraction, analcohol fraction, etc.

It is also known that methanol, acetone and water are commoncontaminants for propylene oxide which are removed only with difficulty.

For example, Mitchell et al. U.S. Pat. No. 2,550,847 is directed to aprocess for separating purified propylene oxide from a crude propyleneoxide product contaminated with acetaldehyde, methyl formate, methanol,etc., by treating the crude mixture with an aqueous basic substancefollowed by recovery of the purified propylene oxide by any suitablemeans such as by decantation. Mitchell et al. reported a recovery of aproduct containing 78 to 82 wt. % of propylene oxide which, they stated,could be increased in purity to about 95 to 99% by fractionaldistillation.

Robeson et al. U.S. Patent No. 2,622,060 discloses a process for thepurification of propylene oxide contaminated with impurities, includingmethanol, by subjecting the impure propylene oxide to distillation inthe presence of an extractive distillation agent comprising an aqueoussolution of an alkali. The inventors report in Example 1 of their patenta method wherein 500 parts by weight of a crude propylene oxide fractionwas extractively distilled in accordance with their invention to obtain325 parts by weight of a product containing about 99.6 wt. % ofpropylene oxide. Thus, a significant loss of propylene oxide occurredduring the process.

In a process unrelated to the purification of propylene oxide, Goddin etal. in U.S. Pat. No. 2,751,337 disclose a process for separating acetonefrom a mixture of acetone with methanol and methyl acetate utilizingwater as an extractive distillation agent.

Hamlin et al. in U.S. Pat. No. 3,409,513 disclose the hydro-extractivedistillation of mixtures comprising acetone, lower aliphatic alcoholsand esters of lower aliphatic alcohols with carboxylic acids. It ispointed out by the patentees that acetone, methyl acetate and methanolform an azeotrope boiling at 55.5°-56.5° C. Hamlin et al. propose torecover partially purified acetone from such a terniary azeotrope byliquid-liquid extraction with water followed by hydro-extractivedistillation of the aqueous phase in order to obtain a partiallypurified acetone fraction.

Washall U.S. Pat. No. 3,578,568 discloses a process for removingoxygen-containing impurities such as acetone, acetaldehyde and methanolfrom impure propylene oxide using a glycol such as ethylene glycol orpropylene glycol as an extractive distillation agent.

Hoorl and Newman U.S. Pat. No. 3,632,482 is directed to a propyleneoxide recovery process by extractive distillation using analcohol-ketone-hydrocarbon solvent. The invention relates to a methodfor the purification of crude propylene oxide contained in a mixtureproduced by the epoxidation of propylene with an organic hydroperoxideand calls for extractive distillation of the crude propylene oxide in aplurality of successive extractive distillation zones with the aid of asolvent mixture consisting essentially of hydrocarbons, alcohols, and/orketones corresponding to the organic hydroperoxide employed in producingthe propylene oxide. In the preferred embodiment of their invention, theextractive distillation agent is a recycle fraction from a three columndistillation sequence wherein the bottoms from the third distillationcolumn are flashed to obtain an overhead composed of hydrocarbons,alcohols and/or ketones which is recycled as an extractive distillationagent to the three distillation columns involved in the propylene oxidepurification sequence.

Burns et al. U.S. Pat. No. 3,715,284 discloses a process for thepurification of impure propylene oxide using acetone or a mixture ofacetone with methanol which is introduced into a distillation columneither below or together with the impure propylene oxide.

Schmidt U.S. Pat. No. 3,881,996 is directed to a distillation sequenceemploying at least three and optionally four columns for thepurification of crude propylene oxide, one of the columns optionallybeing an extractive distillation column wherein a hydrocarbon such asoctane is used as the extractive distillation agent.

Schmidt U.S. Pat. No. 4,140,588 discloses a process for the purificationof propylene oxide contaminated with methanol and acetone using water asan extractive distillation agent, the water being introduced into thedistillation column above the point of introduction of the crudepropylene oxide feed.

Schmidt states at column 2, lines 50-55 that: "Propylene oxide, however,has a substantial solubility in water and is readily hydrolyzed topropylene glycol (PG) in the presence of large amounts of water"--i.e.,in the reboiler section of the tower.

U.S. Pat. No. 4,971,661 discloses the use of an aqueous acetoneextraction to remove methanol from propylene oxide.

U.S. Pat. No. 3,578,568 discloses the use of glycols or glycol ethers inan extractive distillation to remove oxygen containing impurities suchas acetone, acetaldehyde, and methanol. It is claimed that theconcentration of the solvent in the vapor space in the extractivedistillation zone of the distillation tower is preferably between 15 and50 mole percent of the total vapor.

Compared to U.S. Pat. No. 3,578,568, this invention uses considerablylower concentrations of solvent in the extractive distillation zone toremove water and oxygen-containing impurities such as acetone. Since theconcentration of the extractive distillation solvent is lower, the sizeand heat requirements of the associated solvent regenerator are reduced.

U.S. Pat. No. 4,140,588 discloses the use of water in an extractivedistillation to remove methanol and acetone.

U.S. Pat. No. 3,607,669 discloses the use of a C₈ to C₁₂ hydrocarbon toseparate propylene oxide from water.

Shih et al. U.S. Pat. No. 5,000,825 discloses the purification ofmonoepoxides such as propylene oxide that are contaminated withoxygenated impurities such as water, low molecular weight alcohols, lowmolecular weight ketones, low molecular weight aldehydes and the like bythe extractive distillation of the contaminated monoepoxide using alower glycol containing 2 to 4 carbon atoms. Examples of lower glycolsthat are given in the patent include ethylene glycol, 1,2-propane diol,1,3-propane diol, 1,4-butane diol, 1,2 butane diol, 1,3-butane diol and2,3-butane diol. It is stated that higher diols or higher glycol ethersdo not provide sufficient selectivity for the removal of such impuritiesand are not included as the extractive distillation solvents suitablefor use in the invention.

SUMMARY OF THE INVENTION

In accordance with the present invention, an impure propylene oxidefeedstock contaminated with 0.01 to 2 wt. % of water, from about 50 toabout 4,000 ppm of methanol and from about 0.01 to about 2 wt. % ofacetone is charged to the lower half of an extractive distillationcolumn containing at least about 10 theoretical plates and an extractivedistillation agent consisting essentially of 2-methyl-2,4-pentanediol ischarged to the tower at a point at least 4 stages above the impurepropylene oxide feed point. Preferably, the extractive distillationtower will contain from about 30 to about 120 theoretical plates and theextractive distillation agent will be charged to the tower at a point offrom 7 to 50 theoretical stages above the impure propylene oxide feedpoint. The extractive distillation agent is introduced into theextractive distillation column in the ratio of said feedstock to saidextractive distillation agent of from about 0.5:1 to about 20:1, morepreferably 2:1 to 10:1, and most preferably 5:1, whereby a lightdistillate fraction is obtained consisting essentially of propyleneoxide contaminated with significantly reduced amounts of water, methanoland acetone, such as about 5 to about 600 ppm of water, about 15 to2,000 ppm of methanol and about 0.1 to about 100 ppm of acetone.

BACKGROUND OF THE PRESENT INVENTION

When propylene is reacted in liquid phase with an organic hydroperoxidesuch as tertiary butyl hydroperoxide in solution in a solvent such astertiary butyl alcohol in the presence of a soluble epoxidation catalystsuch as a molybdenum epoxidation catalyst, a reaction mixture is formedcomprising propylene oxide, an alcohol corresponding to the organichydroperoxide feedstock and impurities including water and otheroxygenated impurities such as methyl formate, acetaldehyde, acetone andmethanol.

Propylene oxide is a hygroscopic substance, so that water is removedonly with difficulty. It is important to remove as much of the water aspossible, however, because the water present in the propylene oxide willtend to react with the propylene oxide to form propylene glycol.

It is also important to reduce the level of other oxygenatedcontaminants such as methanol and acetone to the lowest reasonablyattainable level.

In accordance with conventional practice, an epoxidation reactionproduct formed by the molybdenum-catalyzed reaction of propylene oxidewith tertiary butyl hydroperoxide in solution in tertiary butyl alcoholis separated into the principle components by distillation so as to formdistillation fractions including a propylene distillation fraction, apropylene oxide distillation fraction, a tertiary butyl alcoholdistillation fraction and a heavy distillation fraction containing themolybdenum catalyst and other products and by-products of theepoxidation reaction. However, the distillation fractions that arethus-obtained are characterized by the inclusion of impurities and,normally, must be further treated if commercially acceptable productsare to be obtained. This is especially true for a propylene oxidedistillation fraction contaminated with water and oxygenatedcontaminants including water, methanol, acetone, etc.

It has been surprisingly discovered in accordance with the presentinvention that substantially all of the water initially present in acontaminated propylene oxide feedstock can be removed therefrom when thepropylene oxide feedstock is extractively distilled in the presence ofan extractive distillation agent consisting essentially of2-methyl-2,4-pentanediol. Even more surprising is our discovery thatsubstantially all of the acetone and most of the methanol present in thecontaminated feedstock can also be removed from the propylene oxide whenusing 2-methyl-2,4-pentanediol as the extractive distillation agent.

BRIEF DESCRIPTION OF THE DRAWING

The drawing is a schematic flow sheet with conventional parts omittedshowing the general recovery sequence that is used in accordance withthe present invention in purifying propylene oxide.

In the drawing, for convenience, the present invention is illustrated inconnection with a process wherein the propylene oxide is prepared by theepoxidation of propylene with tertiary butyl hydroperoxide in solutionin tertiary butyl alcohol to provide a reaction product comprisingpropylene oxide and additional tertiary butyl alcohol.

DESCRIPTION OF THE PREFERRED EMBODIMENT

Turning now to the drawing, there is shown a schematic flow sheetillustrating a preferred method of practicing the process of the presentinvention. In the drawing, conventional parts such as valves, pumps,temperature sensors, pressure sensors, heaters, coolers, flow controlregulation apparatus, etc., have been omitted.

In accordance with a preferred embodiment of the present invention,propylene oxide is separated in a preliminary distillation zone (notshown) from other components of an epoxidation reaction mixture in orderto provide an impure propylene oxide fraction contaminated withoxygen-containing impurities such as acetone, methanol, water, etc.

The impure propylene oxide feedstock that is thus obtained in thepreliminary distillation zone is then purified in a propylene oxidepurification distillation zone, which in accordance with the preferredembodiment of the present invention, comprises two distillation columns,each of which is equipped with an appropriate reflux condensing meansand an appropriate reboiler heating means.

In accordance with the present invention, an impure propylene oxidefraction contaminated with from about 50 to about 4,000 ppm of methanol,from about 0.01 to about 2 wt. % of acetone and about 0.01 to about 2wt. % of water and other oxygen-containing impurities is charged by wayof a line 120 leading to a distillation column 100 which, in accordancewith the present invention, will preferably be a column containing atleast about 10 theoretical plates and more preferably, from about 30 toabout 100 theoretical plates. The column 100 is suitably operated underdistillation conditions including a pressure of about 10 to 40 psia, areflux ratio of from about 2:1 to about 10:1, a reboiler temperaturewithin the range of about 100° to about 250° C. (e.g., 210° C.) and atop temperature of about 20° to about 80° C. (e.g., about 20° C.).

The impure propylene oxide is preferably charged to the distillationcolumn 100 in the lower half thereof. An extractive distillation agentconsisting essentially of 2-methyl-2,4-pentanediol is charged to theupper half of the distillation column 100 by an extractive distillationcharge line 106.

Essentially anhydrous purified propylene oxide containing about 100 ppmor less of water is removed from the column 100 as a light distillationfraction 112, the purified propylene oxide in the line 112 containingsignificantly reduced amounts of methanol and acetone, such as about 15to 900 ppm of methanol and about 0.1 to 100 ppm of acetone. A heavierfraction 110 is withdrawn from the distillation column 100 whichcontains substantially all of the extractive distillation agent chargedby the line 106 and also substantially all of the water, acetone andother oxygen-containing impurities introduced into the column 100 withthe impure propylene oxide 120.

The heavier distillation fraction 110 from the column 100 comprisingwater, methanol, acetone, tertiary butyl alcohol and other impuritiesand extractive distillation agent is charged to a second distillationcolumn 200 wherein light impurities such as methanol, acetone, water,etc., are separated overhead as a distillation fraction 204 that isdischarged from the system for any suitable use, such as for use as asteam boiler feedstock or for recovery.

A heavier distillation fraction 106 is discharged from the distillationcolumn 200 comprising 2-methyl-2,4-pentanediol which is recycled todistillation column 100 by line 106.

2-Methyl-2,4-pentanediol is a compound having the formula: ##STR1##

2-Methyl-2,4-pentanediol is a colorless liquid that is completelymiscible with water and many organic solvents. It has a boiling point of197° C., a specific gravity of 0.9235 (20/20° C.) and a flash point of93.9° C. Technical grades of 2-methyl-2,4-pentanediol will contain waterand if introduced directly into column 100 would add to the level ofcontaminants introduced into the column. In accordance with the presentinvention, fresh 2-methyl-2,4-pentanediol, either as the originalcharge, or as make-up solvent, is introduced into the system by a branchline 230 leading to the charge line 110 for the second distillationcolumn 200 so that any water introduced into the system with the fresh2-methyl-2,4-pentanediol will be separated therefrom in the column 200and withdrawn from the column 200 through the line 204.

EXAMPLES

The invention will be further illustrated by the following specificexamples which are given by way of illustration and not as limitationson the scope of this invention. Where parts are mentioned, they areparts by weight.

Comparative distillation runs were made without the use of an extractivedistillation solvent and with 2-methyl-2,4-pentanediol as the solvent.The results are tabulated below. The runs were conducted in a 25 actualplate pilot plant distillation column at an operating pressure of 14-18psia.

The runs were made on a semi-batch basis wherein an actual known mass ofimpure propylene oxide was charged to a round-bottomed flask that wasattached to the bottom of the Oldershaw-type distillation column. Thedistillation column had a feed line to the top tray. A solvent feed tankon a scale was connected to the feed line. A swinging bucket type ofreflux splitter was placed on the top of the distillation column and acondenser was placed in the conventional manner over the refluxsplitter.

Each run was begun by heating the propylene oxide in the round-bottomedflask until the propylene oxide was boiling up the entire length of thecolumn. The column was maintained in total reflux as a controlled amountof solvent was allowed to flow into the column through thge feed line.The solvent was permitted to flow through the feed line for a shortperiod of time before beginning to take material off. The reflux ratiowas set at 15/5 at a controlled take off rate so as to maintain aconsistent ratio of solvent feed rate to overhead take off rate. The runwas continued until 80 wt. % of the original propylene oxide had beentaken off as overhead. The overhead and bottoms were then analyzed fortheir composition.

In the case where no solvent was used, the procedure was the same exceptthat no solvent was fed through the feed line to the top tray.

    ______________________________________                                                   Feed    Solvent  Overhead Bottoms                                  Component  wt %    wt %     wt %     wt %                                     ______________________________________                                        No Solvent: 6272-2                                                            Water      0.170   N/A      0.0600   1.390                                    MeOH       0.201   N/A      0.0969   N/An                                     Acetone    0.511   N/A      0.0017   N/An                                     No Solvent: 6272-3                                                            Water      0.170   N/A      0.0784   2.1                                      MeOH       0.201   N/A      0.1057   N/An                                     Acetone    0.511   N/A      0.0006   N/An                                     2-Methyl-2,4-pentanediol: 6272-13                                             Water      0.170   0.06     0.0240   0.235                                    MeOH       0.201   N/An     0.0005   N/An                                     Acetone    0.511   N/An     0.0005   N/An                                     Solvent:PO Ratio = 1.1                                                        2-Methyl-2,4-pentanediol: 6272-13                                             Water      0.180   N/A      0.0225   0.046                                    MeOH       0.212   N/An     0.0277   N/An                                     Acetone    0.751   N/An     0.0008   N/An                                     Solvent:PO Ratio = 0.45                                                       ______________________________________                                         Note:                                                                         ND is not detected                                                            N/A is not applicable                                                         N/An is not analyzed                                                     

These distillations demonstrate that the 2-methyl-2,4-pentanediolsolvent improves the separation of methanol and water from propyleneoxide.

Water and oxygen-containing impurities such as acetone and methanol aredifficult to remove from propylene oxide by standard distillation. Theuse of extractive distillation columns with 2-methyl-2,4-pentanediol asthe solvent improves the separation of these impurities from propyleneoxide.

Having thus described our invention, what is claimed is:
 1. Anextractive distillation process for the distillation of an impurepropylene oxide feedstock in a distillation column to remove oxygenatedcontaminants, including water, methanol, and acetone from the impurepropylene oxide feedstock which comprises the steps of:introducing saidimpure propylene oxide feedstock into said distillation column at a feedpoint in the lower half of said distillation column, introducing anextractive distillation agent consisting essentially of2-methyl-2,4-pentanediol into said distillation column at a feed pointabove the said impure propylene oxide feed point, said extractivedistillation agent being introduced into said distillation column in theratio of said impure propylene oxide feedstock to said extractivedistillation agent of from about 1:1 to about 20:1, withdrawing alighter distillate fraction from said distillation column consistingessentially of essentially anhydrous propylene oxide contaminated withreduced quantities of acetone and methanol, withdrawing a heavierdistillation fraction from said distillation column containingsubstantially all of the 2-methyl-2,4-pentanediol, water and acetone anda portion of the methanol introduced into said distillation column.
 2. Aprocess as in claim 1 wherein the impure propylene oxide feedstockcomprises propylene oxide contaminated with from about 0.01 to about 2wt. % of water, from about 50 to about 4000 ppm of methanol and fromabout 0.01 to about 2 wt. % of acetone.
 3. An extractive distillationprocess for the distillation of an impure propylene oxide feedstock in adistillation column to remove oxygenated contaminants, including water,methanol, and acetone from the impure propylene oxide feedstock whichcomprises the steps of:charging said impure propylene oxide feedstock toa distillation column containing at least 25 theoretical plates at afeed point in the lower half of said distillation column, said impurepropylene oxide feedstock comprising propylene oxide contaminated withfrom about 0.01 to about 2 wt. % of water, from about 50 to about 4000of ppm methanol and from about 0.01 to about 2 wt. % of acetone,introducing an extractive distillation agent consisting essentially of2-methyl-2,4-pentanediol into said distillation column at a feed pointat least 4 theoretical plates above the said impure propylene oxide feedpoint, said extractive distillation agent being introduced into saiddistillation column in the ratio of said impure propylene oxidefeedstock to said extractive distillation agent of from about 1:1 toabout 20:1, withdrawing an overhead distillate fraction from saiddistillation column consisting essentially of essentially anhydrouspropylene oxide contaminated with reduced quantities of acetone andmethanol, and withdrawing a bottoms distillation fraction from saiddistillation column containing substantially all of the2-methyl-2,4-pentanediol water and acetone and a portion of the methanolintroduced into said distillation column.
 4. An extractive distillationprocess for the distillation of an impure propylene oxide feedstock in adistillation column to remove oxygenated contaminants, including water,methanol, and acetone from the impure propylene oxide feedstock whichcomprises the steps of:introducing said impure propylene oxide feedstockinto said distillation column at a feed point in the lower half thereof,said distillation column containing at least 25 theoretical plates, saidimpure propylene oxide feedstock comprising propylene oxide contaminatedwith from about 0.01 to about 2 wt. % of water, from about 50 to about4000 ppm of methanol and from about 0.01 to about 2 wt. % of acetone,introducing an extractive distillation agent consisting essentially of2-methyl-2,4-pentanediol into said distillation column at a point atleast 4 theoretical plates above the said impure propylene oxide feedpoint, said extractive distillation agent being introduced into saiddistillation column in the ratio of said impure propylene oxidefeedstock to said extractive distillation agent of from about 1:1 toabout 20:1, fractionating said impure propylene oxide feedstock in saiddistillation column under distillation conditions including a pressureof about 10 to about 40 psia, a reflux ratio of from about 1:1 to about5:1, a reboiler temperature within the range of about 100° to about 250°C. and a top temperature of about 20° to about 80° C. withdrawing anoverhead purified propylene oxide distillate fraction from saiddistillation column consisting essentially of essentially anhydrouspropylene oxide, said purified propylene oxide distillate fraction beingcontaminated with significantly reduced quantities of methanol andacetone, and withdrawing a heavier distillation fraction from saiddistillation column containing not more than 1 wt. % of the propyleneoxide charged to said distillation column and containing substantiallyall of the 2-methyl-2,4-pentanediol water, and acetone and a portion ofthe methanol introduced into said distillation column.